During operation of such a vortex flow sensor, a Karman vortex street is formed downstream of the bluff body. Its pressure fluctuations are converted by a vortex-sensing element into an electric signal whose frequency is proportional to the volumetric flow rate.
Cylindrical bluff bodies in vortex flow sensors were already described and investigated in the early days of such sensors, as shown in U.S. Pat. No. 3,720,104, for example. However, the technical development turned away from cylindrical bluff bodies toward triangular and trapezoidal ones, probably because the measurement accuracy attainable with the former was too low.
Only recently has the cylindrical bluff body been taken up again, as shown in JP-A 8-5419 with English abstract, which describes a vortex flow sensor for measuring the flow velocity and/or the volumetric flow rate of a fluid flowing through a measuring tube, said vortex flow sensor comprising:
a cylindrical bluff body of circular section mounted in the measuring tube and having a smooth surface; PA1 three parallel through bores evenly distributed over the height of the bluff body, whose respective axes are parallel to the direction of flow; and PA1 five vortex-sensing elements, three of which are mounted in the bluff body and are each connected with a respective one of the through bores, while the two others are mounted opposite each other at the surface of the bluff body along an axis perpendicular to the direction of flow and to the longitudinal axis of the bluff body. PA1 a cylindrical bluff body of circular section mounted in the measuring tube and having a surface which is, at least in part, roughened; and PA1 a single vortex-sensing element responsive to pressure fluctuations caused by the vortices.
The purpose of this design is, on the one hand, to increase the measurement accuracy at low flow velocities by averaging the signals from the three vortex-sensing elements mounted in the bluff body and, on the other hand, to utilize the signals from the two vortex-sensing elements positioned at the surface of the bluff body at high flow velocities, thus permitting measurements over a wide flow-velocity range.